1. Field of the Invention
The present invention relates to a method and system for controlling TDMA (Time Division Multiple Access) timing in a satellite communication network and, particularly, to a satellite communication network in which a plurality of land earth stations and a number of terminal earth stations are connected through a satellite system.
2. Description of the Related Art
A satellite communication system has been known in which a plurality of land earth stations (LES's) are connected through a communication satellite having a plurality of electric communication wave beams (referred to as merely "beams", hereinafter) to a number of terminal earth stations (TES's) to provide communication channels between the LES's and the TES's. As an example of the satellite communication system, a mobile satellite communication system for performing communication services for mobile terminals will be described.
FIG. 1 is a schematic block diagram of the mobile satellite communication system, in which there are a number of TES's 3 for each of a plurality of beam areas 4, which can be covered by one beam. A mobile link 6 constructed with a plurality of beams is formed between each beam area 4 and a communication satellite 1.
A plurality of LES's 2 are further provided and each LES 2 and a public switched telephone network (PSTN) 8 are connected to each other through a gate way 230. A feeder link (land earth station channel) constructed with a plurality of beams is provided between each LES 2 and the communication satellite 1.
The communication satellite 1 includes an antenna 101 for the feeder link 7, a duplexer (DPX) 102, a low noise amplifier (LNA) 103, a divider (DIV) 104, a frequency converter (F/C) 105, a traffic channel divider (TCD) 108, a baseband switch (BS) 109, a traffic channel Combiner (TCC) 110, a beam former (BF) 114 and a beam antenna 115.
A forward link is formed by these components and a return link is formed by a return TCD 113, a return BS 112, a return TCC 111, a return combiner 107 and a high power amplifier (HPA) 106.
FIG. 2(A) schematically shows a frequency assignment in the satellite communication system shown in FIG. 1. There are frequencies for the feeder link (FL) 7 and frequencies for the mobile link (ML) 6. The frequency for the feeder link 7 is divided to an uplink frequency FLu and a downlink frequency FLd and the frequency for the mobile link (ML) is divided to an uplink frequency MLu and a downlink frequency MLd.
The communication satellite 1 functions as a repeater for converting the uplink frequency FLu to the downlink frequency MLd in the forward communication channel and converting the uplink frequency MLu to the downlink frequency FLd in the return communication channel.
Since, in the mobile satellite communication, the antenna gain and the transmission power in the land earth station are severely limited, it is necessary to compensate for the limitation in antenna gain and transmission power by increasing the antenna gain and the transmission power in the communication satellite. In order to achieve the compensation, it is usual for the communication satellite to use an antenna having a radius as large as possible and to use a mobile link which utilizes a multi-beam covering a wider area.
Although the feeder link usually uses a single beam, it is effective to miniaturize the land earth station to use a multi-beam instead of the single beam.
In a signal multiplexing system, the time division multiple access (TDMA) system such as shown in FIG. 2(B) has been used mainly for the reasons that the DPX of the terminal station becomes unnecessary and that a control channel during communication can be monitored easily. Since, in the TDMA system, burst signals from a plurality of different stations are transmitted through a single frequency channel, it is necessary, in order to avoid collision between the burst signals, to control the transmission timing in the respective transmission stations.
In order to establish the burst signal synchronization for the TDMA system, a circuit shown in FIG. 3 which is of the global beam system has been used in each land earth station. The global beam circuits each shown in FIG. 3 of respective land earth stations can receive signals transmitted by themselves.
The circuit shown in FIG. 3 includes a land earth station antenna 211, a duplexer (DPX) 212, a high power amplifier (HPA) 213, an up-converter (U/C) 214, a modulator 215, a reference oscillator 222, a demodulator 227, a down-converter (D/C) 228 and a low noise amplifier (LNA) 229.
The circuit further includes a baseband processor (RX) 231, a timing error detector 232, a transmission timing generator 233 and a transmission burst generator 234.
The principle of the burst synchronization is simple. That is, as shown in FIGS. 2(B) and 2(C), each land earth station receives a TDMA signal including a burst signal transmitted by itself from a communication satellite, measures a time difference between a reference burst transmitted by the reference station and the burst signal transmitted by itself and detects the timing error on the basis of a deviation the time difference from its nominal setting value. Then, an output timing of the transmission timing generator 233 is controlled such that the timing error is corrected.
Such burst synchronizing method is known as the closed loop control method. This method can easily establish the synchronization by the simplest negative feedback control, although the correction frequency is limited in a delay time in two ways between the communication satellite and the land earth station. In the above-mentioned method, however, it is clear that it is effective in the case of a global beam system in which the land earth station can receive a signal transmitted by itself.
Incidentally, PR, UW and DATA shown in FIGS. 2(B) and 2(C) represent a preamble portion, a unique word for synchronization and data, respectively.
In a case where the feeder link which is a transmission channel of land earth stations uses multi beam, the respective land earth stations can not always receive burst signals transmitted by themselves and the closed loop control system can not be applied thereto. In such case, means for synchronizing contents of timers of the respective land earth stations becomes necessary.
As an example of such means, a system shown in FIG. 4 is proposed in Japanese Patent Application Laid-open No. Hei 5(1993)-167485. In FIG. 4, reference numerals 41 denotes an antenna, 42 a transceiver circuit, 43 a variable reference clock generator, 44 a D/A converter, 45a.about.45c interfaces, 46 a CPU (arithmetic operation circuit), 47 a GPS antenna, 48 a GPS receiver and 49 a counter.
The system shown in FIG. 4 intends to establish a time synchronization by means of the global positioning system (GPS) receiver which is separate from a satellite communication system, in which a signal from a GPS satellite under control of the U.S. Department of Defense is received through the GPS antenna 47 and the GPS receiver 48, obtains an absolute time from the received signal and controls a frequency of a reference clock A of the reference clock generator 43 according to the absolute time.
As described, the closed loop control method cannot be applied to the usual multi-beam satellite system but to the global beam system for the reason that the measurement of time error is impossible in the multi beam system since a land earth station can not receive a burst signal transmitted by itself.
Further, in the method shown in FIG. 4 in which the GPS system is used, the possibility of using the system is uncertain for the reason that there may be a case where use of the GPS system becomes difficult abruptly when any international trouble including the U.S.A. occurs since the GPS system itself is under control of the U.S. Department of Defense.